1. Field of the Invention
The present invention relates to a method for producing a rare earth element-doped glass which exhibits properties of the rare earth by a sol-gel process.
2. Description of the Related Art
In the production of a glass optical fiber such as quartz glass optical fiber, techniques for producing a highly pure material and controlling optical structure have been almost completed, and the glass optical fiber is practically and widely used.
Since it was found that a glass added with, for example, Nd.sup.3+ ions as active ions could generate a laser beam in the 1960s, it has been confirmed that a glass added with various rare earth elements such as Er.sup.3+, Yb.sup.3+, Ho.sup.3+ or Tm.sup.3+ can also generate a laser beam. This is because the laser transition in the trivalent rare earth element ions is the f-f transition so that it gives a narrow spectrum width in the glass.
The glass has many advantages for practical use, in that it can provide a large size mass compared to a crystalline material, and it is homogeneous and easily processed. In addition, the glass can be easily processed in a fiber form and used to assemble a telecommunication optical fiber system.
Two methods are known for addition of the rare earth element to the glass optical fiber, such as a quartz glass optical fiber, a vapor phase method and a solution impregnating method.
The vapor phase method includes a method comprising heating a chloride of a rare earth element to vaporize it, and introducing the vaporized chloride into a reaction tube together with a quartz glass-forming raw material, such as silicon tetrachloride, in order to form a glass which contains the rare earth element. Another type of vapor phase synthesis is a method comprising heating a mass of silica particles to vitrify it in an atmosphere comprising a vapor of the rare earth elements. This adds the rare earth element to the glass.
Typically, the solution impregnating method comprises dipping a mass of silica particles into a solution of the rare earth element in an alcohol or water, evaporating the alcohol or water and sintering the mass of silica particles to produce a glass containing the rare earth element.
The above conventional methods for producing the rare earth element-doped glass are based on the techniques for producing quartz glass optical fibers and have various merits in quality and productivity. However, by the vapor phase method, the rare earth element is added to the glass in an amount of several ppm to several ten ppm. When the solution impregnating method is used, the rare earth element can be added in an amount up to several thousand ppm. However, the amount of the impregnated rare earth element varies due to the bulk density profile in the mass of silica particles. This makes it difficult to produce a glass in which the rare earth element is uniformly added.